Chromium base alloy



Sept. 24, 196s 'w.H,cHANG 3A0322 CHROMIUM BASE ALLOY United States Patent O Y 3,403,022 CHROMIUM BASE ALLOY Winston H. Chang, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York Filed Oct. 14, 1965, Ser. No. 495,998 2 Claims. (Cl. 75-176) This invention relates to a chromium base alloy and, more particularly, to a chromium base alloy of improved strength particularly in sheet form.

Advances in gas turbine technology for power producing apparatus has identified a need for improved materialsuseful at elevated temperatures. At 18 00 F. and above, some of the currently used iron, nickel or cobalt base superalloys have been found to have limitations for Icertain Iapplications. Therefore, improvement of the refractory metals such as molybdenum, columbium and chromium has been the subject of wide effort. Of these materials, chromium is attractive because of its inherently better oxidation resistance compared with the other refractory metals.

It is a principal object of the present invention to provide 4a chromium vbase alloy of improved high temperature strength and having ductility suiiicient to allow easy reduction from cast to sheet form.

l .Another object is to provide an improved chromium base alloy having improved high temperature strength as well as a recrystallization temperature sufficiently high to further take advantage of work hardening and inhibit embrittlement of the alloy.

These and other objects and `advantages will be more fully understood from the following `detailed `description and examples which are meant to be exemplary of rather than any limitation on the scope of the present invention.

The drawing is =a graphical comparison of tensile properties between a known sheet alloy and that of this invention.

It has been recognized that a chromium base alloy of improved strength can be provided by the addition of a particular a-mount of columbium, in the presence of carbon in certain proportions to control a iine dispersion of columbium carbide. This dispersion within the range of 4this invention strengthens chromium both in the cast as well as in the wrought condition yet the amount of carbon is controlled-to avoid formation of the embrittling chromium carbide phase.

` In one form, the alloy of the present invention consists essentially of, by weight, (S-0.15% C.; 0.5-1.5% Cb; ODS-0.2% Y; 0.05.0.2% 'Ih; up to 0.2% Zr; with the balance essentialy chromium and incidental impurities.

Elements of the Periodic Table of Elements in Group IV-A, of which zirconium is typical, and Group V-A, of which columbium is typical, have been broadly specified for 4addition to some -of the refractory metals, particularly columbium and molybdenum. However, in the case of chromium as a base, it has been -recognized that all types of carbide dispersions do not play the same role in strengthening.

The relationship of alloying additions such as the Group IV-A and Group V-A metals along with carbon in chromium base alloys is particularly significant. The fiow and fracture behavior of chromium is profoundly affected by interstitials. The arrangements, size and type of carbide has been found to affect strength, ductility and air oxidation in-difi'erent ways. The influence of processing and heat treatment in controlling these kinds of mechanisms and hence the properties is significantly important in chromium base alloys. For example, in the presence of about 0.1 weight percent carbon in a chromium base, the element zirconium will form a carbide precipitate significantly different from that formed through the addiice tion of columbium. The zirconium carbide, because of its relatively low solubility in chromium, forms in the ascast condition a carbide network in the grain boundaries. Columbium has agreater solubility in chromium than does zirconium. Hence in the presence of carbon, columbium will precipitate as a fine rather than coarse dispersion of columbium carbide throughout the matrix as well as in the grain boundary of the chromium base structure. Thus the columbium addition will provide greater strengthening to the Valloy because of the formation of a fine precipitate different in kind than that of zirconium. Furthermore, it has been found that whereas chromium base :alloys strengthened with columbium carbide tend to be brittle in the recrystallized condition, in the stress relief conditions they have significantly improved strength. By maintaining the addition of columbium and its relationship to carbon within the range of 0.5-1.5 weight percent columbium and 0.05-(M5 -weight percent carbon, the recrystallization temperature can be maintained sufiiciently high, at least at about 2300 F.

Like a number `of other chromium base alloys, it has been found that the alloy -of the present invention can be further improved to resistance to oxidation `and nitrification through the retention of yttrium and thorium such as in the range of about (M15-0.2% Y and (LOS-0.2% Th. Below these amounts there is insufficient yttrium and thorium retained in the alloy either to getter the interstitials or to afford a significant amount of improvement in air oxidation resistance. Above those stated amounts, the alloy of the present invention tends to become embrittled.

One of the most useful forms of the alloy of the present invention is as a sheet material because of its high strength combined with good ductility and `good air oxidation resistance particularly up to 1800 F. Because its processability is good, it is readily reduced into sheet fro-m `a cast form.

As was indicated above, the range of columbium included within the alloy of the present invention is 0.5-1.5 weight percent. The addition of amounts in excess of about 1.5% Cb allows the formation of too much CbCr2 which is a coarse intermetallic phase. CbCr2 tends to lower the melting point of the alloy and is detrimental to oxidation resistance. On the other hand, the inclusion of less than about 0.5 weight percent columbium in the presence of an excess of carbon allows the formation of the embrittling chromium carbide-such as Cr23C6 rather than the fine columbium carbide. Thus the carbon range is controlled to be within the range of about 0.05-015 weight percent. The inclusion of too much of either columbium or carbon outside the range of the present invention leads to the formation of massive carbides. Unlike the fine carbide dispersion found in the present invention, the massive carbides lead to lower strength. In addition, the excess of columbium or carbon increases aging kinetics which has a significantly adverse effect on mechanical properties.

Typical of the alloys within the scope of the present invention are those listed in the following Table I in which compositions of the Cb/C atomic ratio is in the particularly beneficial range of 1-3 to l.

TABLE I.WT. PERCENT, BALANCE Cr Example Zr Cb C Y Th the early rejection of zirconium carbide by the chromium matrix upon solidiication. Thus any stabilization of the columbium carbide through the use of zirconium is probably limited to the zirconium solubility in chromium. Therefore, the alloy of the present invention can include up to `about `0.2 weight percent zirconium without signiicantly adverse elect on the mechanical properties of the alloy through the formation of the more massive zirconium carbide.

The alloy compositions shown in Table I were induction melted under argon to insure homogeneity and to minimize ingot cracking. The alloy was cast into a 3" diameter Y2O3-stabilized zirconia Crucible. The yttrium and thorium additions in the charges were about 6 and 2 times, respectively, larger than the nominal contents to compensate for their losses through scavenging effects. Therefore the percentages for those elements listed in the specification refer to the retained amount rather than the amount added.

In order to provide sheet material for subsequent testing, the casting was extruded into bar before rolling into sheet. In order to elect carbide dissolution to allow control of dispersion either during extrusion or by postextrusion heat treatment, higher extrusion temperatures were required. However, the extrusion temperature should not exceed 2800 F. to avoid incipient fusion. Therefore, a temperature of 2700 F. was selected for initial extrusion.

The extruded bar was first reduced 50% in thickness at about 2000 F. followed by rolling in the range of about 1500-1800 F. to produce the sheets of 0.05 thick material.

The rolled sheets were stress relieved at 2000 F. for 1 hour after which physical properties were determined.

The following Table II lists the tensile properties of the rolled sheet material in the stress relieved condition,

TABLE II.-TENSILE PROPERTIES [Rolled sheet, stress relieved (2,000" F./1 hr.)]

Tem Ultimate 0.2% yield Elongation Example F. Strength strength (Percent) (K s.i.) (K s i.)

A comparison between the alloy of the present invention and a known sheet alloy reported to be one of the best available based on chromium is shown in the drawing. The known alloy has a composition, by weight, of 93.5% chromium, 0.5% titanium and 6% magnesium oxide. Both sheet alloys were at the same thickness of about 50 mils. The signicantly better strength of th alloy of the present invention is shown by the drawing.

As was mentioned above, the alloy of the present invention is particularly suitable for use in an air oxidizing atmosphere. The good oxidation and nitrication resistance is shown by the data of Table III.

TABLE rrr-10o HR. OXIDATION DATA Weight gain (mgJcm) Unlike alloys dispersion strengthened with more massive carbide such as zirconium carbide, the effect on ox-v idation resistance of reduction from cast to wrought condition is not great. The hour oxidation tests conducted to obtain the data of Table III were performed on bar specimens of 0.22 x 0.35 x 0.5 specimens in the rolled condition. The specimens were prepared by grinding and polishing through 400 grit paper followed by water and alcohol rinsing. Specimens were placed in zirconia crucibles and oxidized continuously in a tubular furnace with natural air convection.

Although the present invention has been described in connection with specific examples, it will be recognized by those skilled in the metallurgical art the modifications and variations of which the invention is capable and which are intended to be covered by the appended claims.

What is claimed is:

1. A chromium base alloy of improved strength consisting essentially of, by weight, 0.5-1.5% Cb; 0.05- 0.l5% C; 0.05-0.2% Y; 0.05-0.2% Th; up to about 0.2% Zr; with the balance chromium and incidental impurities.

2. A chromium base alloy of improved strength consisting essentially of, by weight, about 1% Cb; about 0.1% C; 0.050.2% Y; 0.05-0.2% Th; up to about 0.2% Zr; with the balance chromium and incidental impurities.

References Cited UNITED STATES PATENTS 2,955,937 10/1960 McGurty et al. 75-176 3,011,889 12/1961 Baranow 75-176 3,137,572 6/1964 Aronin et al 75-176 3,174,853 3/1965 Sims et al 75-176 3,208,847 9/1965 FOX 75-176 3,227,548 1/1966 Clark 75-176 FOREIGN PATENTS 923,039 4/ 1963 Great Britain.

CHARLES N. LOVELL, Primary Examiner. 

1. A CHRONIUM BASE ALLOY OF IMPROVED STRENGTH CONSISTING ESSENTIALLY OF, BY WEIGHT, 0.5-1.5% CB; 0.050.15% C; 0.05-0.2% Y; 0.05-0.2% TH; UP TO ABOUT 0.2% ZR; WITH THE BALANCE CHROMIUM AND INCIDENTAL IMPURITIES. 